This invention relates to vulcanized synthetic rubber compositions. The considerations relating to vulcanizing a predominantly natural rubber (NR) composition are different from those for vulcanizing synthetic rubber (SR). For example, SR is essentially free of a proclivity to reversion upon vulcanization with sulfur. Tire tread requires a vulcanizate which is oxidation stable, highly durable under driving conditions, along with good wet and dry traction, low rolling resistance, inter alia, yet with sufficient flexibility to withstand deformation during use. Increasing the amount of sulfur decreases stability to oxidation (also refered to as `oxidation` or `oxidative` stabililty) when used with known accelerators in conventional ratios. As a result, the tire manufacturing industry has progressively lowered sulfur levels and increased the amount of accelerator used.
Vulcanization of rubber with sulfur is a complicated process which is not fully understood. A variety of chemical compounds ("products" to avoid confusion with tread compounds) are formed depending upon the type of rubber, the sulfur level, the accelerator and activator used, etc. For example, unaccelerated sulfur vulcanization produces long polysulfide crosslinks approaching 50 S atoms per crosslink. Relatively low concentrations of sulfenamide accelerators produce "long" crosslinks containing from about 3 to about 10 sulfur atoms per chain which is referred to as a "polysulfide" chain or crosslink. Sulfur donor systems give even shorter crosslinks--about 3 S atoms per chain. Relatively high concentrations produce short crosslinks containing 1 or 2 sulfur atoms per chain, referred to as monosulfide and disulfide chains or crosslinks, respectively. Finally, a "sulfurless" cure system which uses only accelerators which are sulfur donors, gives about 1.5 S atoms per chain.
Besides crosslinks or chains, a variety of other products such as cyclic sulfides and disulfides, conjugated dienes and trienes, and pendant accelerator groups attached through sulfur to the backbone, are also formed. Each of these affects the physical properties of the vulcanizate. Although relationships between physical properties of vulcanizates and manv of these compounds have been intensively studied, the relationships and the resulting properties are unpredictable.
Because the proportionate amount of each compound formed in the vulcanizate depends so heavily on the type of rubber used, the type of sulfur, the type and thermal history of the carbon black, in addition to so many other factors, for example the sulfur level, whether oil extended, the type of accelerators and sulfur donors used, the amount of pigments and fillers such as silica, if these are used, inter alia, it is well accepted that it is implausibe to make logical deductions with reasonable assurance from measurements of physical properties obtained with a particular cure system and rubber compound, from properties obtained with a similar cure system but a different type of rubber.
More particularly this invention relates to the use of a combination of compounds from the classes of benzothiazole sulfenamide (BTS) and thiocarbamyl sulfenamide (TCS) accelerators each of which has good scorch safety, but the TCS is more efficient. By "efficient" I mean that TCS provides the same density of crosslinks with a lesser amount (than BTS) in the same or less time. The improved efficiency (about 30% better) of the TCS results in the formation of a crosslink network which is "shorter" than the network produced by BTS. The vulcanizate therefore has different properties. TCS improves thermal and oxidative stability of SBR vulcanizates and also gives lower permanent set and heat buildup. However TCS adversely affects tear strength and flex life.
My goal was to take advantage of the efficiency of the TCS in combination with the BTS in such a way as to use a lower net amount of accelerators and to increase the amount of sulfur.
The goal of Carpino in U.S. Pat. No. 4,119,588 (hereafter the '588 patent) was to overcome the proclivity to reversion in a NR tread compound. He succeeded in doing so, but when he used more than 2.0 phr (parts per hundred parts of rubber) of sulfur he found that he sacrificed oxidative stability. He therefore prescribed using less than 1.5 phr sulfur for maximum reversion resistance. He failed to realize that this poor stability was due to the predominantly NR compound he was working with. Instead of oxidative stability he found that TCS, or a combination of TCS and BTS, provided the reversion resistance he sought in the NR vulcanizate, because of the short crosslinks produced. Normally, short crosslinks provide improved oxidation stability but they decrease the oxidation stability of NR if TCS is used. The oxidation stability of NR vulcanized with BTS and TCS (which produces short crosslinks) is poorer than when the NR is vulcanized with BTS alone (which produces longer crosslinks).
The oxidation stabilitv of NR with a combination of TCS, or TCS and BTS gets progressively worse as the sulfur level is increased. Carpino was unaware that poor oxidation stability was not due solely to the level of sulfur. Since he worked only with a natural rubber system, he did not learn enough about the effect of the type of tread rubber to enable him to reach any conclusion about the effect of the same accelerators in a SR system. In particular, he missed discovering that the same combination of TCS and BTS accelerators, in a particular range of ratios defining a window of applicability, produces the opposite effect (namely better oxidation stability). Though he used too much combined accelerators for a SR tread compound, this had no bearing on his appreciation for the problem of using too much accelerator since he was in a NR compound. The disclosure of the '588 patent is incorporated bv reference thereto as if fully set forth herein.
The same goal of using a high sulfur level challenged Ahagon et al who were concerned with a SR tread compound and provided a solution in U.S. Pat. No. 4,309,318. Though they obtained low rolling resistance in tires with high (3.2.-5.0 phr) sulfur content in a predominantly SBR tread rubber, the oxidative stability was sacrificed as was to be expected for reasons which are well known (see "Oxidation Hardening of "SBR" by M. L. Studebaker and J. R. Beatty, Rubber Chem. Technol., 45, 450 (1972). They made no attempt to maintain the modulus constant (see data in Table 2 of U.S. Pat. No. 4,309,318). Substantially constant is a necessary criterion for making a comparison of tread performance in the present framework of reference.
Polysulfide crosslinks imbue the vulcanizate with high tensile strength, improved flex resistance and tear strength, high heat buildup and compression set, and poor thermal and oxidative stability. Good oxidative and thermal stability, low tensile strength, heat buildup and compression set, poor flex resistance and tear strength are attributable to monosulfide and disulfide crosslinks. Thus, a tire tread composition is always a compromise. This invention provides an unexpectedly good one.